I thought police might have taken the mixer at some point for forensic examination.
That location looks very different from ground level and a different angle from the pictures I've previously seen. I think it's the same place after looking at the whole video, but I did wonder.
Regarding searching through dry, sandy dirt: many tools would be appropriate. The mixer would be handy to find things of a significant higher density than the dirt and bigger chunks. It could also quickly wash caked dirt off of chunky things. A screen/sifter would be ideal for finer exploration.
Regarding the landscape: I have noticed that the arial views of the property make it seem much flatter than it appears from eye level.
I find myself trying to imagine what the property looked like at dusk and after dark, with LE arriving. I imagine that, depending on the moon phase (appears to have been 1/4 moon approx.) it would be really dark looking from the house into the distance, and looking upon the lights on in the house/from vehicles from the distance would have looked really bright and obvious. I imagine that after dark with the lights on in the house, one could see their way back to the house from quite a distance.
I try to think what it was like for LE when they first arrived. If Gus had wandered off, it would have been easier to find him after dark because he would, I think, have had the lights to orient him, and would probably head home if he were moving (rather than injured and immobile). UNLESS, it is much less flat than I think, and of course Gus is shorter than adults, making hills that much bigger in comparison.
If someone deliberately harmed him, it probably happened in daylight. If the 5:00, 5:30 times are fiction, they had to have been deliberately chosen for some reason. Were there a lot of phone calls at 5:30 that needed to be explained? Was 5:30 the time all adults would be reunited in the evening? Was it truly the typical routine, that Gus would play at that time, then come back for a bath, dinner and wind down for the evening?
LE keeps emphasizing that the parents are not suspects.
Shannon acting more or less alone theory: 5:30 was chosen because it was the latest time possible. That's when Josie and/or Jess would come home and it would be known by more than Shannon that Gus was missing shortly after 5:30. If that is accurate, it means only Shannon could be the one who gave the suspicious shifting time lines, which I suppose is plausible regarding the rest of the day. It means only one person was responding to Gus' disappearance deceptively when LE appeared. BUT: It does not make any sense at all for Shannon to kill her grandchild she could simply evict instead, nor does it make sense for her to cover for an accident.
Josie acting with Shannon: 5:30 could be the time chosen because soon after that time, Jess would know Gus is missing. The inconsistent time lines could be either or both grandparents. For example, if Josie and Jess gave contradictory time lines it could be something like both said Josie went back to get sandwiches for herself and Jess, but Jess said it took much longer than Josie said it took. BUT: again it does not make any sense for either grandparent to kill a child they could evict, and covering up an accident is even harder to imagine with two people present.
After an accident, the energy would go to making it explainable to the mother, not LE, IMO.
Edited to add Josie acting mostly alone: I doubt that highly because for some reason, there appears to be almost no sign of Gus. For Josie to have killed, hidden and cleaned the evidence all while also pretending to search for Gus from 5:30 forward sounds farther fetched to me than her being involved with some clean up help at an earlier time. Also, since the time frame is so short, something would have been a big clue in retrospect to someone. She immediately changed her clothes instead of immediately searched? Something like that after a few days would have come to Shannon and/or Jess' mind,
Then: I have trouble believing that neither Jess nor Josh, being unaware that Shannon and/or Josie murdered Gus, would not become convinced that he was kidnapped. What else would they think, if they did not suspect each other or the grandparents? In my controversial opinion, failure to put up flyers around town and use social media (or have volunteers do so on their behalf) means that they had ruled out abduction prematurely. Or if not prematurely, with extreme logic that I don't think I could muster up in the circumstances.
MOO